Where is the denoise option in 2.90?
The denoiser options are in the Properties Window &gt; Render settings &gt; Sampling &gt; Denoising.

You can choose what kind of denoiser to use.

NLM is the original denoiser (native non-local denoiser), that runs on any computing device.
OpenImageDenoiser is Intel Open Image Denoiser, that runs on CPU
In this section you can enable the viewport denoising as well.